Mobile stations (MS), also known as mobile terminals, wireless terminals and/or User Equipment (UE) are enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two mobile stations, between a mobile station and a regular telephone and/or between a mobile station and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The mobile stations may further be referred to as mobile telephones, cellular telephones, iPads, laptops or any other similar device with wireless capability. The mobile stations in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a Radio Base Station (RBS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, or relay node, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. The base stations communicate over the air interface operating on radio frequencies with the mobile stations within range of the base stations.
In some radio access networks, one or more base stations may be connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC) e.g. in Universal Mobile Tele communications System (UMTS). The Radio Network Controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
When a mobile station desires to access a wireless communication system such as e.g. a GSM, a WCDMA or an LTE system, it begins by sending random-access bursts to the nearest/strongest Radio Base Station. Depending on the distance to the Radio Base Station the bursts will arrive at the Radio Base Station more or less out of sync with the TDMA-frame structure of the Radio Base Station. To get the Mobile Station aligned with the TDMA frames the initial response from the GSM system contains a Timing Advance (TA) value. The Timing Advance value informs the Mobile Station of how much earlier the Mobile Station must transmit its bursts for them to arrive well synchronized to the Radio Base Station.
In a wireless communication system, or cellular mobile system as it also may be referred to, performing a handover (HO) from one cell to another is a critical task. When a handover decision is taken it is desirable that the handover time is as short as possible. Thus the handover is to be made as soon as possible following the reception of the Radio Resource Control (RRC) message triggering the handover, which may be before confirming successful reception Hybrid Automatic Repeat request/Automatic Repeat reQuest (HARQ/ARQ) of this message.
Considering X2 handover, FIG. 1 shows the control plane signalling. The RRC message triggering the handover is the RRC connection reconfiguration message, see Message 7 in FIG. 1. At reception of this message the user equipment is to perform the handover by switching to the target network node, or target eNB, as soon as possible. As the note above indicate this can be done by the user equipment without sending neither a Radio Link Control (RLC) status message for the RRC Package Data Unit (PDU) comprising the RRC connection reconfiguration message, nor a HARQ feedback for the transport block comprising the RRC connection reconfiguration message. The above means that the source network node, or source eNB, cannot know if the user equipment has successfully performed the handover until the UE context release message; see Message 17 in FIG. 1 is received from the target network node. This message informs the source network node that the user equipment successfully performed the handover. Until Message 17 is received source network node will, in case the user equipment leaves the cell without sending HARQ acknowledgement (ACK), continue sending HARQ retransmissions and RLC retransmissions trying to reach the user equipment, until predefined retransmission limits has been reached.
If the user equipment neither sends a HARQ feedback or RLC acknowledgement for the RRC message triggering the handover (Message 7 in FIG. 1), the source network node simply perform HARQ retransmissions and RLC retransmissions of the handover-message i.e., the handover-triggering RRC message. If the user equipment already performed the handover to a new cell it will get inter-cell interference from it source cell trying to reach it. This self-caused interference may potentially decrease the handover success rate.
FIG. 2 shows a scenario where the user equipment successfully performs a handover to a new cell while the old cell is still trying to reach the user equipment.
In the scenario, a handover has been made where the source network node (Sector 2) continues to transmit to the user equipment after the user equipment has entered the new cell (Sector 1). The x-axis shows the time in milliseconds. The y-axis shows the efficiency calculated from the modulation and coding scheme, as the code rate times the number of bits per modulation symbol summed for the transport blocks used (triangles). For each triangle there is a corresponding circle showing the reported efficiency, calculated from the reported rank and reported Channel Quality Index (CQI).
FIG. 3A show a scenario where the user equipment receives a handover command from the source network node (eNB), and performs the handover to the target network node, without transmitting any HARQ or RLC acknowledgement, to the source network node of the handover command. Thus the source network node continues re-transmitting the handover command, possibly until a UE Context Release message (message 17 in FIG. 1) is received from the target network node.
FIG. 3B show a scenario similar to the one illustrated in FIG. 3A. However, here the user equipment is sending a HARQ acknowledgement of the handover command to the source network node (eNB), but no RLC acknowledgement, why the source network node anyway continues retransmitting the handover command.
FIG. 3C illustrates a scenario where the user equipment is sending both a HARQ acknowledgement and a RLC acknowledgement of the handover command to the source network node (eNB).
Thus some user equipment, upon receiving a handover command may firstly send acknowledgements before starting the handover. However, some user equipment may start the handover without sending any acknowledgements, so the source network node could not know whether the user equipment did indeed send acknowledgements, but something went wrong in reception/transmission; or if the user equipment did not hear that it had message coming to it, thereby not being able to send any acknowledgements; or if the user equipment has successfully received the handover command and initiates handover without sending any acknowledgements.
Thereby, just in case, the source network node continues re-transmitting the handover command, thereby causing an increased exposure for intra cell interference. Also, by keeping resources scheduled for user equipment which are no longer present in the cell, the transmission capacity of the system is decreased.
However, as a handover typically is performed at the cell edge, where the signal propagation conditions may be somewhat detoriated, the user equipment would experience an increased risk of being disconnected if no retransmissions at all would be performed by the source network node in the described scenario.